Flat-panel x-ray detectors are used increasingly frequently in x-ray imaging systems. A flat-panel x-ray detector consists of 1000 by 1000 to around 3000 by 3000 detector elements under a scintillator, such as cesium iodide for example. The x-rays converted into light by the scintillator are measured in the detector elements and converted into a data value which is then represented as a gray-scale value, so that the individual pixels can be assembled into an x-ray image.
A flat-panel x-ray detector can for example be a solid-state x-ray detector made of amorphous silicon. Such a detector has the property that the signal behavior of each individual detector element is different. This means that different areas of the detector have different sensitivities to x-ray radiation. Gain correction is thus undertaken when an x-ray image is generated in which the data values are corrected by purely computational means. The basis for the correction is the previous measurement of the relevant sensitivity of the individual detector elements. This measurement is undertaken by determining a so-called gain image, with the gain image providing a gain value in each image element or pixel. In the prior art a single gain image is determined for calibration of an x-ray imaging system when the system is commissioned. “Determining” a gain image is taken in this case to mean that a gain image is obtained by recording multiple images, and that a gain image is obtained by recording a plurality of bright images, i.e. images, for which the x-ray radiation arrives at the flat-panel x-ray detector in a defined and direct manner, without an imaging object disturbing the path of the x-ray radiation. This plurality of bright images is recorded without any changes to the x-ray imaging settings and a plurality of dark images is then recorded. An average is then taken from the bright images, i.e. an averaged pixel data value is determined over the various images for each individual pixel. The dark images are images which are recorded without x-ray radiation being emitted at all. The dark images correspond to the signals of the detector elements which emit these signals in any event, i.e. to an offset of the signals. This offset is then derived for each detector element from the associated averaged data value for the bright images and a gain image is determined in this manner. As a rule the data value from the gain image is also standardized to 1. An image entry for an individual pixel of 1,1 means for example that the corresponding detector element reflects the x-ray radiation amplified by a factor of 1,1, so that to record an x-ray image, the data value emitted by this detector element must be divided by 1,1, in order to thus obtain a data value which corresponds to a gain of 1. Conversely a gain value of 0.9 means that the detector element is slightly weaker than the average, and that a corresponding data value of an x-ray image must be divided by 0,9 in order to thus obtain a slightly increased data value which corresponds to a realistic data value for an average detector element.